Polymer networks, swelling behavior

Common methods of surface charaaerization are applied in investigating thin films of responsive polymers. The swelling behavior of a surface-tethered PNlPAAm network in D2O was characterized with neutron refleaion and compared to the dembdng behavior of linear PNlPAAm in solution. Local and global dynamics of the anchored network chains are investigated by fluorescence correlation spectroscopy and photon correlation spectroscopy. ... 

We present a review of theoretical and experimental results on the swelling behavior and collapse transition in polymer gels obtained by our group at Moscow State University. The main attention is paid to polyelectrolyte networks where the most important factor is additional osmotic pressure created by mobile counter ions. The influence of other factors such as condensation of counter ions, external mechanical force, the mixed nature of low-molecular solvents, interaction of network chains with linear macromolecules and surfactants etc. is also taken into account Experimental results demonstrate a good correlation with theoretical analysis. 

The first quantitative theory of the reentrant collapse was developed in Ref. [49], The theory explained the phenomenon of the simple reentrant collapse which was observed in Refs. [14, 41]. A more general theory of swelling and collapse of charged networks in the binary solvent was developed in Ref. [31] and described in Sect. 2.4.1. We have seen that one of the most essential features of the swelling behavior in mixed solvents is a redistribution of solvent molecules within the network giving a different solvent composition in the gel and the external solution. This redistribution is more pronounced for the collapsed gel, because the probability of contacts of the molecules of the solvent with polymer links in the collapsed gel is higher than in the swollen gel. 

Important theoretical and experimental considerations of the use of macromolecular theories for the description of coal network structures have been recently analyzed (1). Relevant equations describing the equilibrium swelling behavior of networks using theories of modified Gaussian distribution of macromolecular chains have been developed by Kovac (2 ) and by Peppas and Lucht (3) and applied to various coal systems in an effort to model the relatively compact coal network structures (1 4). As reported before (1), Gaussian-chain macromolecular models usually employed in the description of polymer networks (such as the Flory... 

Typical spectra obtained for the probed attached at the network junction are shown in Figs. 2 and 3 The probe in THF possesses the highest ratio for CT/LE. An increased intensity for the LE was measured for the swollen sample. This effect can be explained by different polarity /mobility of the probe One can assume that covalent bonded probes possess another probe mobility than free dissolved probe molecules. Furthermore, the covalent bonded probe molecule that shows a higher polarity in comparison to the siloxane chains is located at the network junction. The attached probe molecule is surrounded mainly by siloxane chains of the network. Addition of polar swelling solvents leads to an increase of the CT-emission and the ratio CT/LE is mainly influenced by the composition of polymer and swelling agent (compare spectra for dried and swollen N1 samples in Fig. 2). Therefore, the covalently bonded probe shows another fluorescence behavior in comparison to the free dissolved probes that can be surrounded also by solvent molecules. 

Using Flory-Huggins theory it is possible to account for the equilibrium thermodynamic properties of polymer solutions, particularly the fact that polymer solutions show major deviations from ideal solution behavior, as for example, the vapor pressure of solvent above a polymer solution invariably is very much lower than predicted from Raoult s law. The theory also accounts for the phase separation and fractionation behavior of polymer solutions, melting point depressions in crystalline polymers, and swelling of polymer networks. However, the theory is only able to predict general trends and fails to achieve precise agreement with experimental data. 

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